Research On The Optimization Design For Cooperative Non-Orthogonal Multiple Access Systems

Mobile communication networks are usually updated in a decade with the characteristic of using one generation,building one generation and developing one generation.Nowadays,the mobile communication networks have developed from the first generation(1G)of analog communications to the fifth generation(5G)of broadband communications.5G can satisfy the low-latency,high-reliability and high-data rate communication requirements and support the data transmission in massive Internet of Things(Io T).The industry-university-research community predicts that it is expected to see the commercialization of 6G around 2030.6G aims at designing new architectures,exploring new abilities and creating new technologies’ ecology,in order to promote the development of the society of integrating digital twins with virtual reality.As one of the landmark technologies which represents the upgrading of mobile communication networks,multiple access technologies have great effects on the development of 6G.International telecommunication union has classified non-orthogonal multiple access(NOMA)as one of the key multiple access technologies that can support the massive machine type communications(m MTC).Meanwhile,cooperative communication is a classical diversity technology which acts as one of the essential technologies for promoting the development of mobile communication networks.The application of cooperative communication into NOMA systems is expected to provide high-spectrum efficiency,large-capacity and wide-coverage multiple access schemes for future mobile communication networks.In this context,how to exploit the potential wireless resources and design resource optimization schemes for cooperative NOMA(C-NOMA)systems,is a critical problem to be solved.This dissertation introduces a few novelly wireless communication technologies into C-NOMA system and formulates various optimization schemes to improve the system performance.Firstly,taking advantage of the degree-of-freedom provided by buffer-aided relaying(BR)technology for C-NOMA systems,four optimization schemes are formulated to improve the system throughput efficiently.Assuming that the NOMA power allocation coefficients are fixed,an optimal mode selection scheme is developed and the corresponding theoretical optimal throughput is derived.In order to further improve the system throughput under the constraint of user fairness,two power allocation and mode selection schemes with different complexity are proposed.One is an optimal scheme with a higher complexity and the other one is a suboptimal scheme with a lower complexity.Considering the practical communication conditions that the buffer has finite size and that the users are delay-sensitive,a heuristic throughput-delay aware transmission scheme is proposed.Secondly,friendly jamming(FJ)technology is introduced into C-NOMA system with the purpose of overcoming the security issue caused by the untrusted relay.The NOMA power allocation at base station and transmitting power of friendly jammer are optimized under the constraints of users’ quality of service and required signal-to-interference-plus-noise ratio threshold of friendly jammer.Then,a joint power optimization scheme is proposed for enhancing the system secrecy sum rate efficiently.Considering that the channel state information is imperfect in C-NOMA system,the proposed joint power optimization scheme is extended to improve the system secrecy sum rate.Thirdly,aiming at enhancing the physical layer security performance of C-NOMA system with an internal eavesdropper,the BR and proactive jamming technologies are introduced.Taking into account the constraints of buffer stability,maximum transmitting power and users’ quality of service,the NOMA power allocation,transmitting power and mode selection are optimized.Then,a joint power allocation and mode selection scheme is proposed to significantly improve the secrecy sum rate.Finally,intelligent reflecting surface(IRS)aided communication technology is utilized to the device-and-device(D2D)communication underlaying NOMA system.The transmitting power,phase shifts of IRS elements and decoding orders are optimized under the constraints of user fairness,maximum transmitting power and users’ quality of service.Then,a joint resource optimization scheme is proposed which can enhance the sum throughput efficiently.